In conventional type aeration systems, e.g., in most aeration systems using a micro-bubble generating system installed for culture and growth of aquatic animals, air bubbles are generated by injecting the air under pressure into water through fine pores of tubular or planar micro-bubble generating systems installed in the tank, or air bubbles are generated by introducing the air into water flow with shearing force or by vaporizing the air dissolved in water by rapidly reducing the pressure of the pressurized water.
In the aeration process using the micro-bubble generating system with the above functions, the operation is basically controlled by adjusting the air supply quantity or the number of the micro-bubble generating systems to be installed, while it is necessary to efficiently dissolve gas such as air, carbon dioxide, etc. into water and further to promote circulation of the water.
However, in the aeration system using the conventional type micro-bubble generating system, e.g. a diffusion system based on injection, even when fine pores are provided, when air bubbles are injected under pressure through pores, the volume of each of the air bubbles is expanded, and the diameter of each air bubble is increased to several millimeters due to surface tension of the air bubbles during injection. Thus, it is difficult to generate air bubbles of smaller diameter. Also, there are problems, such as clogging of the pores or increases of power consumption caused by the operation for long time.
In the system to generate the air bubbles by introducing the air into water flow with shearing force using vanes and an air bubble jet steam, it is necessary to have higher number of revolutions to generate cavitation. Also, there are problems of power consumption increase and the problem of corrosion of vanes or vibration caused by the generation of cavitation. Further, there are problems in that only a small amount of micro-bubbles can be generated.
In the system where a gas-liquid two-phase flow collides with the moving vane or projection, fishes or small aquatic animals in natural lakes or culture tanks may be injured, and this causes trouble in the development and maintenance of the environmental condition necessary for the growth of fishes and other aquatic animals.
Further, in the pressurizing system, the system must be designed in larger size and requires higher costs, and operation cost is also high.
In none of the prior art in this field, as described above, has it been possible to generate micro-bubbles with diameters of not more than 20 μm in industrial scale.